In a typical cellular radio system, wireless terminals (also known as mobile stations and/or user equipment units (UEs)) communicate via a radio access network (RAN) to one or more core networks (CN). The radio access network (RAN) covers a geographical area which is divided into cell areas, with each cell area being served by a base station, e.g., a radio base station (RBS), which in some networks may also be called, for example, a “NodeB” (UMTS) or “eNodeB” (LTE). A cell is a geographical area where radio coverage is provided by the radio base station equipment at a base station site. Each cell is identified by an identity within the local radio area, which is broadcast in the cell. The base stations communicate over the air interface operating on radio frequencies with the user equipment units (UE) within range of the base stations.
In some versions of the radio access network, several base stations are typically connected (e.g., by landlines or microwave) to a controller node (such as a radio network controller (RNC) or a base station controller (BSC)) which supervises and coordinates various activities of the plural base stations connected thereto. The radio network controllers are typically connected to one or more core networks.
The Universal Mobile Telecommunications System (UMTS) is a third generation mobile communication system, which evolved from the second generation (2G) Global System for Mobile Communications (GSM). The Universal Terrestrial Radio Access Network (UTRAN) is essentially a radio access network using wideband code division multiple access (WCDMA) for user equipments (UEs), or user equipment units. In a forum known as the Third Generation Partnership Project (3GPP), telecommunications suppliers propose and agree upon standards for third generation networks and UTRAN specifically, and investigate enhanced data rate and radio capacity. Specifications for the Evolved Universal Terrestrial Radio Access Network (E-UTRAN) are defined for the 3rd Generation Partnership Project (3GPP).
The Evolved Universal Terrestrial Radio Access Network (E-UTRAN) comprises the Long Term Evolution (LTE) and System Architecture Evolution (SAE). Long Term Evolution (LTE) is a variant of a 3GPP radio access technology wherein the radio base station nodes are connected to a core network (via Access Gateways(AGWs)) rather than to radio network controller (RNC) nodes. In general, in LTE the functions of a radio network controller (RNC) node are distributed between the radio base stations nodes (eNodeB's in LTE) and AGWs. As such, the radio access network (RAN) of an LTE system has an essentially “flat” architecture comprising radio base station nodes without reporting to radio network controller (RNC) nodes.
The International Telecommunications Union-Radio communications sector (ITU-R) has specified a set of requirements for 4G standards, named the International Mobile Telecommunications Advanced (IMT-Advanced) specification. ITU-R has also stated that Mobile WiMAX and LTE, as well as other beyond-3G technologies that do not fulfill the IMT-Advanced requirements, could nevertheless be considered “4G”, provided they represent forerunners to IMT-Advanced compliant versions and have a substantial level of improvement in performance and capabilities with respect to the initial third generation system.
The nodes and devices, such as base stations and wireless terminals, which participate in wireless communications generally employ a communication interface that typically includes a transmitter and a receiver, and one or more antenna(s) that may connect to both the transmitter and the receiver. In some technologies such as multiple input multiple output (MIMO) one or both of the node(s) and device(s) participating in the wireless communications have plural antenna.
A baseband receiver, which may be found in a wireless terminal or a network node such as a base station in WCDMA technology, may apply an equalizer to compensate for dispersivity of the channel. The dispersivity may be a result of receiving multiple reflections of the same transmitted signal, which may resemble several echoes of a same source. Those reflections are also known as channel “taps”.
The baseband receiver may employ a “path searcher” to find the channel taps. A reference signal sent by the source is generally correlated at the receiver to a known pattern to identify the delays of the different taps. More particularly, the ‘path searcher’ finds the channel taps by integrating energy (i.e., correlation of the known reference and signal and summation) of the continuously transmitted reference signal. The detection of the channel taps decides if the receiver is declared to be in synchronization, e.g., in “sync” state. The receiver, e.g., path searcher, generally detects some of the delays, may miss some of the delays, and may even add extra delays on its own even though it does not detect the reference signal. Since each tap generally fades during the transmission independently of the other taps, some taps will be too weak for the receiver to detect. The delays selected by the receiver are called “fingers”. When the fingers have been selected, the data signal can be decoded at each finger and combined together by an equalizer such as MMSE (Minimum-Mean-Square-Error), Rake or GRake, for example. Generally, the data part of the channel is not processed until the sync-state is achieved.
The procedure of detecting the taps can take time, but the channel taps are generally required to process data transmitted over the channel. Ideally, the path searcher would track the channel continuously in order to catch all channel taps where the energy is present, but in reality this would take or consume more time and resources. Thus the path searcher might not be run continuously, which can lead to missing a channel tap. Furthermore, since the channel is generally changing during the detection procedure, the path searcher may have to estimate the time interval during which the integration is performed. This also can lead to missing some channel taps.
In some networks it is possible to transmit and receive over plural carriers or sub-carriers, e.g., over plural carrier frequencies. In such networks information is typically transmitted over the air interface between base stations and wireless terminal in units such as a frame which is formatted in such a manner to be understood by both the base station and the wireless terminals. In some radio access technologies, a frame (or subframe) is conceptualized as comprising a two dimensional array or “resource grid” of resource elements (RE) The resource elements are generally arranged in symbol order along a first (horizontal) direction and according to frequency subcarrier along a second (vertical) direction.
Two carrier frequencies are considered to be adjacent if they are quite close to each other, i.e., typically 20 MHz from each other. As used herein, the criteria for adjacency is as follows: When a transmitter sends two signals simultaneously, one on each frequency (i.e., one on the primary frequency and one of the adjacent frequency), the channels that each signal will experience will have different fast fading tracks, but the channel taps will most likely be the same or at least belong to the same delay spread interval.
In a multi-carrier network, a same tap may be a strong tap in one of the adjacent carriers but weak in the other of the adjacent carriers. This can result in missing a tap that could become stronger later on. Missing a finger or tap can have the consequence of not being able to reach the targeted SIR (i.e., Signal-to-Interference Ratio) and thus leading to power rushes.